\tikzstyle{cir} = [draw, shape=circle, inner sep=0.1em]
     \begin{tikzpicture}
      \node (a) at (0,0) {A}
      [level distance=4em, sibling distance=3.3em]
      child {node (x1) {$\mathrm{X_{1}}$}}
      child {node (x2) {$\mathrm{X_{2}}$}}
      child {node (dot) {$\mathrm{\ldots}$} edge from parent[draw=none]}
      child {node (xn) {$\mathrm{X_{n}}$}};
 \end{tikzpicture}